This invention relates a vertical continuous annealing furnace, and a method of operation thereof, particularly suited for annealing strip of such special steels as stainless steel.
The conventional tower-type continuous annealing furnace calls for hearth rollers to guide the vertical path of strip running up and down. These hearth rollers are able to withstand temperature of 650.degree. C. to 720.degree. C. with which plain carbon steel strip is treated. In a higher temperature range between 800.degree. C. and 1,000.degree. C. in which stainless steel strip is annealed, however, there arises a problem of scale buildup on the surface of the hearth rolls which might, in turn, produce surface defects on the strip threaded thereover. For this reason, stainless steel strip has been treated in horizontal tunnel-type furnaces, ranging between 10 and 18 meters in length.
In an attempt to offer a solution to this problem, one of the inventors proposed a vertical continuous annealing furnace suited for the annealing of stainless and other similar steel strip in Japanese Patent Application No. 50900 of 1977 (Japanese Patent Public Disclosure No. 135,808 of 1978). This prior invention is characterized by providing a slit-like opening in the furnace-top cover, through which strip is continuously fed into the furnace to form a catenary-like freely hanging loop to which annealing heat is applied. This furnace has no strip-guiding hearth rollers at all. In other words, a vertical furnace dispensing with hearth rollers was the main point of this prior invention.
However, this prior invention was not without problems. When the line stops, strip is held within the furnace in the freely looped state mentioned before, where it is oxidized by an oxidizing atmosphere resulting from the rapid heating of strip and, thereby, giving rise to steel loss and a reduction in its quality. To avoid this, the freely looped strip, while the line is at a standstill, should preferably be lifted and horizontally hung over the charging and discharging pinch rolls. Yet, this cannot be achieved without opening the furnace-top cover, which would expose the interior of the furnace to the outside air. This exposure would cause a heavy heat loss and, in serious cases, the spalling of furnace walls as a result of rapid cooling from a higher temperature level.
The same prior invention gave no particular specification as to the arrangement of heating burners. The description of its preferred embodiments states simply that heating burners are disposed inside the furnace. The attached drawings show the burners disposed in a single vertical row on each side of a vertical partition wall hanging down from the furnace top. Later experiments, however, have shown that this burner arrangement is not satisfactory.
With the vertical partition wall mentioned above integral with the furnace top cover, the furnace according to the prior invention is divided into two sections, the entry and delivery sides, which are thermally cut off from each other.
However, when the furnace is used for soaking heated strip, both the furnace and strip temperatures are in many cases substantially the same on either side of the partition wall. In such cases, there is no need to eliminate the mutual thermal effect of strip between the entry and delivery sides. Rather, the heat buildup by the partition wall leads to a heat loss and a damaging of the furnace's unit heat consumption.
Having enough space inside and, therefore, receiving no pressure from the piece charged therein, vertical continuous annealing furnaces need no strength to withstand any such pressure. Also, their walls should cause as little heat buildup as possible so that their heating pattern may be switched quickly as the type of steel treated changes. For these reasons, it was thought preferable that the furnace wall refractories possess both high thermal resistance and good heat-insulating properties, so ceramic fiber came into popular use. Later again, it was found that the strip traveling through the furnace often hits and damages the furnace walls of ceramic fiber while weaving in a loop or upon falling as a result of breaking.
In the vertical annealing furnaces that have no strip-guiding hearth rollers in the bottom, strip is allowed to turn upward freely drawing a loop, making it necessary to keep the lower end of the free loop at a substantially steady level.
It is also necessary to give special consideration to the emission of combustion gas so that as much uniform heating as possible may be achieved through a further enhancement of the furnace's thermal efficiency.
Where stainless or other special steel is involved, special considerations must be given also to cooling means, since attaining the aimed-for metallurgical structure in such steel calls for more delicate and subtle adjustments than in the case of plain carbon steel.
Where a great treating capacity is required, two or more furnaces must be installed in series since the capacity of one furnace is limited. Such a series arrangement facilitates accelerating the strip treating speed, increasing the production capacity, and applying various heating patterns. Meanwhile it is essential to prevent the escape of heat from the strip traveling from one furnace to another to avoid the waste of energy and minimize the effect of heat loss on steel quality.